As a technique for estimating an electrode wearing rate in spark discharge of a spark plug, Japanese Unexamined Publication No. 2014-17153 discloses a technique for generating spark discharge between electrodes in atmospheric gas, decomposing, for each wavelength, light emitted by the spark discharge, calculating an emission intensity based on the result of a taken image, and estimating an electrode wearing rate according to previously obtained correlation between emission intensities and electrode wearing rates.